Dual Light Emission of CsSnI3-Based Powders Synthesized via a Mechanochemical Process

Lead toxicity has hindered the wide applications of lead halide perovskites in optoelectronics and bioimaging. A significant amount of effort has been made to synthesize lead-free halide perovskites as alternatives to lead halide perovskites. In this work, we demonstrate the feasibility of synthesizing CsSnI3-based powders mechanochemically with dual light emissions under ambient conditions from CsI and SnI2 powders. The formed CsSnI3-based powders are divided into CsSnI3-dominated powders and CsSnI3-contained powders. Under the excitation of ultraviolet light of 365 nm in wavelength, the CsSnI3-dominated powders emit green light with a wavelength centered at 540 nm, and the CsSnI3-contained powders emit orange light with a wavelength centered at 608 nm. Both the CsSnI3-dominated and CsSnI3-contained powders exhibit infrared emission with the peak emission wavelengths centered at 916 nm and 925 nm, respectively, under a laser of 785 nm in wavelength. From the absorbance spectra, we obtain bandgaps of 2.32 eV and 2.08 eV for the CsSnI3-dominated and CsSnI3-contained powders, respectively. The CsSnI3-contained powders exhibit the characteristics of thermal quenching and photoelectrical response under white light.


Introduction
The success in the synthesis of lead halide perovskites at low cost has opened active research fields for the applications of lead halide perovskites in photovoltaics and sensing and light-emitting devices and systems [1,2].One of the challenges for the applications of lead halide perovskites is the lead toxicity, which has hindered the commercialization of lead halide perovskites.This has stimulated extensive work to explore the replacement of lead halide perovskites with lead-free halide perovskites and lead-free halide double perovskites.Sn-based halide perovskites represent one important group of lead-free halide perovskites.
There are various methods available to synthesize Sn-based halide perovskites, including spin-coating [3], vapor-assisted deposition [4], solid-phase sintering [5], hot injection [6], and mechanochemical processing [7,8].Weiβ et al. [9] used atomic layer deposition and pulsed chemical vapor deposition to form γ-CsSnI 3 film on patterned silicon substrates and obtained a bandgap of 1.20 eV from the Tauc plot.Shum et al. [10] used a two-step method to obtain CsSnI 3 films on three different substrates of glass, ceramics, and silicon with thermal and electron-beam evaporators in a vacuum chamber (~10 5 Torr).Kim and Kang [11] combined spin-cast with drop cast to form CsSnI 3 crystals in a glove box with nitrogen.Wang et al. [12] used a one-pot process to synthesize CsSnI 3 nanocrystals with and without the use of an antioxidant solvent additive (TPPi) and the prepared CsSnI 3 nanocrystals to form CsSnI 3 films.Kapil et al. [3] used a spray-deposition method to prepare Cs 2 SnI 6 films and obtained a bandgap of 1.54 eV.Murshed and Bansal [13] applied drop-coating to construct Cs(Sn,Pb)I 3 films with a bandgap of 1.5 eV for the applications in perovskite-based solar cells.Nairui et al. [14] synthesized Cs 2 SnI 6 powders via a one-step method and studied dependence of the optical characteristics of the Cs 2 SnI 6 powders on the process parameters, including reactant type and solvent type.Jiang et al. [15] used a chemical bath method to prepare Cs 2 SnI 6 powders and reported the long-term stability of the Cs 2 SnI 6 powders over a period of one month.Lee et al. [16] used electrospraying to form Cs 2 SnI 6−x Br x layers and evaluated the Br effect on the bandgaps of the Cs 2 SnI 6-x Br x layers.Saparov et al. [17] prepared Cs 2 SnI 6 films via a two-step process and obtained a bandgap of 1.60 eV.Tang et al. [18] obtained Cs 2 SnI 6 with a size-dependent light emission via an aqueous process.It is worth noting that extensive work has been conducted on the applications of Cs-, Sn-, and Sn-Pb-based perovskites in the field of solar cells.There are few works on infrared emission and green emission of Sn-based halide perovskites.Also, most processes reported in the literature to prepare Sn-based halide perovskites are much more complex than the mechanochemical process used in this work.
Currently, few studies [8,19] are on the use of mechanochemical processing to synthesize Sn-based halide perovskites.To realize the potential applications of Sn-based halide perovskites in solar cells and bioimaging, we used a mechanochemical method to synthesize CsSnI 3 -based powders under ambient conditions from CsI and SnI 2 powders.In contrast to the work reported in the literature, the temporal evolution of the optical characteristics of the prepared CsSnI 3 -based powders were characterized.The prepared CsSnI 3 -based powders were divided into two groups-one is CsSnI 3 -dominated powders, which emit green light under the excitation of UV light of 365 nm, and the other is CsSnI 3 -contained powders, which emit orange light under the excitation of UV light of 365 nm.Both the CsSnI 3 -dominated powders and CsSnI 3 -contained powders exhibit infrared emission with the emission wavelengths centered at 916 nm and 925 nm, respectively, under a laser of 785 nm in wavelength.
For the synthesis of CsSnI 3 -dominated powders, 0.4 mmol (103.92mg) CsI was mixed with 0.4 mmol (149 mg) SnI 2 to form a mixture without any additives and water.The mixture was put in a ceramic mortar and ground mechanically at ~18.4 • C and with a humidity of ~22% for 5 min initially.Twenty milliliters of DI water was then added to the ground material, which was further ground mechanically for 5 min, leading to the formation of black powders (CsSnI 3 -dominated powders) under white light, as shown in Figure 1a.The as-prepared black powders were stored in a vacuum chamber (Napco 5831) at 44.7 KPa for one week.
For the synthesis of CsSnI 3 -contained powders, 0.2 mmol (51.96 mg) CsI was mixed with 0.2 mmol (74.5 mg) SnI 2 with 200 µL of DI water.The mixture was put in a ceramic mortar and ground mechanically at ~18.4 • C and with a humidity of ~22% until the color of the mixture became light black.The ground mixture was dried naturally to form light black powders (CsSnI 3 -contained powders) in the mortar under white light, as shown in Figure 1b.
The crystal structures of the prepared CsSnI 3 -based powders were analyzed on an X-ray analyzer (Bruker D8 Discover, Bruker, Billerica, MA, USA) with a scintillation counter detector under the CuKα radiation of λ = 1.5406Å.The chemical compositions and morphologies of the prepared CsSnI 3 -based powders were characterized on a scanning electron microscope (SEM) (JEOL JSM-5900lLV, JEOL, Tokyo, Japan) equipped with an EDS microanalysis system.humidity of ~22% for 5 min initially.Twenty milliliters of DI water was then added to ground material, which was further ground mechanically for 5 min, leading to formation of black powders (CsSnI3-dominated powders) under white light, as shown Figure 1a.The as-prepared black powders were stored in a vacuum chamber (Napco 58 at 44.7 KPa for one week.

Results and Discussion
Figure 1 depicts optical images of the synthesized CsSnI 3 -dominated powders, which were stored in a vacuum chamber, and the prepared CsSnI 3 -contained powders, which were placed under ambient conditions, under UV light of 365 nm in wavelength over a period of 7 days.It is evident that the color of the CsSnI 3 -based powders under the UV light experienced gradual changes from light green to dark green for the CsSnI 3 -dominated powders and from light orange to dark orange for the CsSnI 3 -contained powders over the period of 7 days.The green emission of CsSnI 3 -based powders under UV light of 365 nm in wavelength has not been reported in the literature to our knowledge.Note that Bharti et al. [20] recently reported the formation of green CsSnI 3 via solid state reaction under mechanical grounding and sintering at 280 K.The orange emission of the CsSnI 3 -contained powders is qualitatively in accord with the light emission of Cs 2 SnI 6 nanobelts/nanocrystals in mother liquor reported by Wang et al. [21].Note that the Cs 2 SnI 6 nanobelts/nanocrystals reported in the work of Wang et al. [21] were synthesized by a hot injection method with the use of octadecene, oleylamine, and oleic acid, which is significantly different from the method used in this work.
Figure 2a-d depicts SEM images of green-emitting powders.There are plate-like structures formed from the grounding of the mixture, as shown in Figure 2a,b, which can be attributed to the combinational effect of the shear and compression deformation.There are small particles on the surfaces of the plate-like structures.The SEM images in Figure 2c,d show the formation of aggregates of particles in a nearly octahedral shape and in a nearly spherical shape, respectively.The size of the particles in a nearly octahedral shape is in a range of 100 nm to 300 nm, and the size of the particles in a nearly spherical shape is in a range of 380 nm to 880 nm.The EDS analyses, as shown in Figures S1-S3 in Supplementary Information, yield atomic ratios of nearly 1:1:3 of Cs:Sn:I, 1:1:3 of Cs:Sn:I, 2:1:6 of Cs:Sn:I, and 1:1 of Cs:I for the plate-like structures, the particles on the surface of the plate-like structures, the particles in a nearly octahedral shape, and the particles in a nearly spherical shape, respectively.This result indicates that the prepared green-emitting powders consist of CsSnI 3 , Cs 2 SnI 6 , and CsI crystals.Note that the octahedral shape of Cs 2 SnI 6 crystal is consistent with the observation reported in the literature [18,22,23].
and 157 nm to 587 nm in diameter.The EDS analyses, as shown in Figures S4-S6 Supplementary Information, yield atomic ratios of nearly 1:1 of Cs:I, 2:1:6 of Cs:Sn:I, an 1:1:3 of Cs:Sn:I for the particles in a bean-like or sphere-like shape, the particles in octahedral shape, and the rod-like particles, respectively.This result indicates that t prepared orange-emitting powders consist of CsSnI3, Cs2SnI6, and CsI crystals.Figure 3 presents the XRD patterns of the freshly prepared CsSnI3-dominat powders and CsSnI3-contained powders and the corresponding ones stored for one wee According to Figure 3a, there are diffraction peaks corresponding to CsSnI3, Cs2SnI6, Sn and CsI for the freshly prepared CsSnI3-dominated powders, which are in accord with t EDS results presented in the supplementary information.Specifically, the diffracti peaks centered at 25.52°, 25.90°, 31.48°,37.73°, 41.36°, and 46.78° correspond to the crys planes of (131), (211), (221), (051), (002), (171), and (152) of orthorhombic CsSnI3 (PDF#9 001-4070); the diffraction peaks centered at 26.53°, 30.73°, and 44.02° correspond to t crystal planes of (222), (400), and (440) of cubic Cs2SnI6 (PDF#97-025-0743); the pe centered at 48.80° corresponds to the crystal plane of (211) of cubic CsI (PDF#97-004-493 and the diffraction peak centered at 28.67° corresponds to the crystal plane of (3 11) monoclinic SnI2 (PDF#97-000-2831).Using the XRD peaks, the molar fractions individual compounds in the freshly prepared CsSnI3-dominated powers are calculat and listed in Table 1.It is evident that CsSnI3 has the largest molar fraction of 32.80 Thus, the green-emitting powders are referred to as CsSnI3-dominated powders.T larger molar fraction of SnI2 than CsI in the freshly prepared powders indicates that mo CsI reacted to form CsSnI3 and Cs2SnI6 than SnI2 during the grinding.It is speculated th the green emission of the powders may be associated with the doping in SnI2.  Figure 2e-h depicts SEM images of the orange-emitting powders.There are many particles presented in a bean-like or sphere-like shape from the grounding of the mixture, as shown in Figure 2e,f, in contrast to the plate-like structures for the green-emitting powders.The SEM images in Figure 2g,h show particles in an octahedral shape and in a rod shape, respectively.The size of the particles in a bean-like or sphere-like shape is in a range of 250 nm to 880 nm, and the size of the particles in an octahedral shape is in a range of 500 nm to 1 µm.The dimensions of the rod-like particles are 500 nm to 4 µm in length and 157 nm to 587 nm in diameter.The EDS analyses, as shown in Figures S4-S6 in Supplementary Information, yield atomic ratios of nearly 1:1 of Cs:I, 2:1:6 of Cs:Sn:I, and 1:1:3 of Cs:Sn:I for the particles in a bean-like or sphere-like shape, the particles in an octahedral shape, and the rod-like particles, respectively.This result indicates that the prepared orange-emitting powders consist of CsSnI 3 , Cs 2 SnI 6 , and CsI crystals.
Figure 3 presents the XRD patterns of the freshly prepared CsSnI 3 -dominated powders and CsSnI 3 -contained powders and the corresponding ones stored for one week.According to Figure 3a, there are diffraction peaks corresponding to CsSnI 3 , Cs 2 SnI 6 , SnI 2 , and CsI for the freshly prepared CsSnI 3 -dominated powders, which are in accord with the EDS results presented in the supplementary information.Specifically, the diffraction peaks centered at 25.52 • , 25.90 • , 31.48 • , 37.73 • , 41.36 • , and 46.78 • correspond to the crystal planes of (131), (211), ( 221), ( 051), (002), (171), and (152) of orthorhombic CsSnI 3 (PDF#97-001-4070); the diffraction peaks centered at 26.53 • , 30.73 • , and 44.02 • correspond to the crystal planes of ( 222), (400), and (440) of cubic Cs 2 SnI 6 (PDF#97-025-0743); the peak centered at 48.80 • corresponds to the crystal plane of (211) of cubic CsI (PDF#97-004-4938); and the diffraction peak centered at 28.67 • corresponds to the crystal plane of ( -311) of monoclinic SnI 2 (PDF#97-000-2831).Using the XRD peaks, the molar fractions of individual compounds in the freshly prepared CsSnI 3 -dominated powers are calculated and listed in Table 1.It is evident that CsSnI 3 has the largest molar fraction of 32.80%.Thus, the green-emitting powders are referred to as CsSnI 3 -dominated powders.The larger molar fraction of SnI 2 than CsI in the freshly prepared powders indicates that more CsI reacted to form CsSnI 3 and Cs 2 SnI 6 than SnI 2 during the grinding.It is speculated that the green emission of the powders may be associated with the doping in SnI 2 .Fresh One week The XRD pattern of the CsSnI3-dominated powders stored in a vacuum chamber one week is also depicted in Figure 3a.The diffraction peaks centered at 25.43°, 25.8 27.317.31°,31.48°,37.82°, and 46.70° correspond to the crystal planes of (131), (211), (22 (051), (002), and (152) of orthorhombic CsSnI3 (PDF#97-001-4070); the diffraction pea centered at 13.15°, 26.58°, 30.89°, 44.13°, 52.38°, and 54.62° correspond to the crystal plan of (111), ( 222), (400), (440), (622), and (444) of cubic Cs2SnI6 (PDF#97-025-0743); the pe centered at 48.84° corresponds to the crystal plane of (211) of cubic CsI (PDF#97-004-493 and the diffraction peak centered at 28.87° corresponds to the crystal plane of (3 11) monoclinic SnI2 (PDF#97-000-2831).There are no diffraction peaks of new pha presented in the figure in comparison with the freshly prepared one.Using the XR peaks, the molar fractions of individual compounds in the CsSnI3-dominated powd stored in the vacuum chamber for one week are calculated and listed in Table 1.It evident that there are slight decreases in the molar fractions of both CsSnI3 and SnI2 a increases in the molar fractions of both Cs2SnI6 and CsI.The increase of Cs2SnI6 is lik from CsSnI3 and SnI2 with the following reactions: There is a decrease in the molar fraction of SnI2 over the period of 7 days, suggesti that the doping effect becomes weaker, as indicated by the optical images in Figure Note that the presence of oxygen in the vacuum chamber was due to the limit of t vacuum pressure.
Following the method used by Misra et al. [24], the pseudo-cubic lattice constants the freshly prepared and the stored CsSnI3-dominated powders are calculated from t XRD patterns in Figure 3a and listed in Table S1 in Supplementary Information.It evident that the lattice constants of CsSnI3 and Cs2SnI6 increase slightly after being stor The XRD pattern of the CsSnI 3 -dominated powders stored in a vacuum chamber for one week is also depicted in Figure 3a.The diffraction peaks centered at 25.43  222), (400), (440), (622), and (444) of cubic Cs 2 SnI 6 (PDF#97-025-0743); the peak centered at 48.84 • corresponds to the crystal plane of (211) of cubic CsI (PDF#97-004-4938); and the diffraction peak centered at 28.87 • corresponds to the crystal plane of ( -311) of monoclinic SnI 2 (PDF#97-000-2831).There are no diffraction peaks of new phases presented in the figure in comparison with the freshly prepared one.Using the XRD peaks, the molar fractions of individual compounds in the CsSnI 3 -dominated powders stored in the vacuum chamber for one week are calculated and listed in Table 1.It is evident that there are slight decreases in the molar fractions of both CsSnI 3 and SnI 2 and increases in the molar fractions of both Cs 2 SnI 6 and CsI.The increase of Cs 2 SnI 6 is likely from CsSnI 3 and SnI 2 with the following reactions: There is a decrease in the molar fraction of SnI 2 over the period of 7 days, suggesting that the doping effect becomes weaker, as indicated by the optical images in Figure 1a.Note that the presence of oxygen in the vacuum chamber was due to the limit of the vacuum pressure.
Following the method used by Misra et al. [24], the pseudo-cubic lattice constants of the freshly prepared and the stored CsSnI 3 -dominated powders are calculated from the XRD patterns in Figure 3a and listed in Table S1 in Supplementary Information.It is evident that the lattice constants of CsSnI 3 and Cs 2 SnI 6 increase slightly after being stored for one week.
The XRD pattern of the freshly prepared CsSnI 3 -contained powders is depicted in Figure 3b.The peaks centered at 26.54 • , 30.90 • , 44.31 • , 52.44 • , and 54.99 • correspond to the crystal planes of (222), (400), ( 440 2. It is evident that CsI has the largest molar fraction of 73.63%.Thus, the orange-emission powders are referred to as CsSnI 3 -contained powders.220), ( 222), (400), ( 440), (622), and (444) of cubic Cs 2 SnI 6 (PDF#97-025-0743); the peaks centered at 27.44 • and 48.64 • correspond to the crystal planes of ( 110) and (211) of cubic CsI (PDF#97-004-4938); the peak centered at 25.52 • corresponds to the crystal plane of (131) of orthorhombic CsSnI 3 (PDF#97-001-4070); and the peak centered at 28.39 • corresponds to the crystal plane of ( -311) of monoclinic SnI 2 (PDF#97-000-2831).There are no diffraction peaks of new phases presented in the figure in comparison with the freshly prepared one.Using the XRD peaks, the molar fractions of individual compounds in the CsSnI 3 -contained powders stored under ambient conditions for one week are calculated and listed in Table 2.It is evident that there are slight decreases in the molar fractions of both CsSnI 3 and SnI 2 , a large decrease in the molar fraction of CsI, and a large increase in the molar fraction of Cs 2 SnI 6 .There are reactions that lead to the increase of Cs 2 SnI 6 and the decrease in CsI in the CsSnI 3 -contained powders.
From Table 2, we note a large molar fraction of CsI in the CsSnI 3 -contained powders.The orange-emission of the CsSnI 3 -contained powders is likely associated with the doping of Sn in CsI, as illustrated in Figures S7-S9 in Supplementary Information.
The larger molar fraction of CsI compared to SnI 2 in the CsSnI 3 -contained powders indicates that more SnI 2 reacted to form CsSnI 3 and Cs 2 SnI 6 than CsI.The orange emission of the powders can be attributed to the doping in CsI.After one week, the molar fraction of Cs 2 SnI 6 increases, while the molar fractions of SnI 2 , CsSnI 3 , and CsI decrease, suggesting that the Cs 2 SnI 6 comes from CsSnI 3 and SnI 2 through the reactions illustrated in Equations (1) and (2).The decrease in the molar fraction of CsI means the doping effect is weaker as indicated by the optical images in Figure 1b.
Figure 4a,b presents PL spectra of the freshly prepared CsSnI 3 -based powders under UV light of 365 nm and a laser of 785 nm, respectively.It is interesting to observe that the CsSnI 3 -dominated powders emit both green light centered at 540 nm in wavelength under the UV light and infrared light centered at 916 nm in wavelength under the 785 nm laser.The emission of the green light centered at 540 nm has not been reported in the literature, and the emission of the infrared light centered at 916 nm corresponds to a bandgap of 1.35 eV in accord with 1.31 eV reported by Chung et al. [25].
the visible and infrared regimes, which can be attributed to the differences in compositions.Figure 5a depicts the absorbance spectrum of the CsSnI3-dominated powders.There is no distinct band-edge absorption peak, which might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5a.From the Tauc plot, we obtain a bandgap of 2.32 eV, which is slightly larger than 2.29 eV from the PL spectrum of the CsSnI3-dominated powders under UV light of 365 nm and less than 2.55 eV reported by Chung et al. [25], Figure 5b depicts the absorbance spectrum of the CsSnI3-contained powders.There is no distinct band-edge absorption peak, which is similar to the CsSnI3-dominated powders.Such behavior again might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5b.From the Tauc plot, we obtain a bandgap of 2.08 eV, which is slightly larger than 2.04 eV from the PL spectrum of the CsSnI3-contained powders under the excitation of UV light of 365 nm.The long-term stability of the CsSnI3-based powders was examined over a period of 7 days.Figure 6a,b depicts the PL spectra of the CsSnI3-dominated powders and CsSnI3contained powders, respectively.The emission wavelengths of both the powders remain unchanged.The peak intensity decreases continuously, which is consistent with the color According to Figure 4a,b, the CsSnI 3 -contained powders emit orange light centered at 608 nm in wavelength under the UV light and infrared light centered at 925 nm in wavelength under the laser.The emission of the orange light centered at 608 nm is close to the PL emission of Cs 2 SnI 6 centered at 620 nm reported by Wang et al. [21], who prepared Cs 2 SnI 6 nanocrystals with a hot-injection process.The emission of the infrared light centered at 925 nm corresponds to a bandgap of 1.34 eV in accord with 1.32 eV reported by Wang et al. [26].The PL peak intensity for the green emission is significantly lower than that for the orange emission, and the PL peak intensity for the infrared emission centered at 916 nm is higher than that for the infrared emission centered at 925 nm.Such differences suggest that the CsSnI 3 -dominated powders (green emission) and CsSnI 3 -contained powders (orange emission) possess different characteristics in terms of photoluminescence in the visible and infrared regimes, which can be attributed to the differences in compositions.
Figure 5a depicts the absorbance spectrum of the CsSnI 3 -dominated powders.There is no distinct band-edge absorption peak, which might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5a.From the Tauc plot, we obtain a bandgap of 2.32 eV, which is slightly larger than 2.29 eV from the PL spectrum of the CsSnI 3 -dominated powders under UV light of 365 nm and less than 2.55 eV reported by Chung et al. [25], Materials 2024, 17, x FOR PEER REVIEW 7 of 13 the visible and infrared regimes, which can be attributed to the differences in compositions.Figure 5a depicts the absorbance spectrum of the CsSnI3-dominated powders.There is no distinct band-edge absorption peak, which might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5a.From the Tauc plot, we obtain a bandgap of 2.32 eV, which is slightly larger than 2.29 eV from the PL spectrum of the CsSnI3-dominated powders under UV light of 365 nm and less than 2.55 eV reported by Chung et al. [25], Figure 5b depicts the absorbance spectrum of the CsSnI3-contained powders.There is no distinct band-edge absorption peak, which is similar to the CsSnI3-dominated powders.Such behavior again might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5b.From the Tauc plot, we obtain a bandgap of 2.08 eV, which is slightly larger than 2.04 eV from the PL spectrum of the CsSnI3-contained powders under the excitation of UV light of 365 nm.The long-term stability of the CsSnI3-based powders was examined over a period of 7 days.Figure 6a,b depicts the PL spectra of the CsSnI3-dominated powders and CsSnI3contained powders, respectively.The emission wavelengths of both the powders remain unchanged.The peak intensity decreases continuously, which is consistent with the color Figure 5b depicts the absorbance spectrum of the CsSnI 3 -contained powders.There is no distinct band-edge absorption peak, which is similar to the CsSnI 3 -dominated powders.Such behavior again might be due to the contributions from four different compounds and the wide range of crystal sizes.Using the absorbance spectrum, the corresponding Tauc plot is constructed and shown as an inset in Figure 5b.From the Tauc plot, we obtain a bandgap of 2.08 eV, which is slightly larger than 2.04 eV from the PL spectrum of the CsSnI 3 -contained powders under the excitation of UV light of 365 nm.
The long-term stability of the CsSnI 3 -based powders was examined over a period of 7 days.Figure 6a,b depicts the PL spectra of the CsSnI 3 -dominated powders and CsSnI 3 -contained powders, respectively.The emission wavelengths of both the powders remain unchanged.The peak intensity decreases continuously, which is consistent with the color change, as shown in Figure 1.Such a trend implies that the prepared CsSnI 3based powders are not at a "stable" state.There exist chemical reactions occurring in the CsSnI 3 -based powders over the period of 7 days, as supported by the changes in the molar fractions shown in Tables 1 and 2. The chemical reactions lead to the evolution of individual compounds and result in the decrease in the PL peak intensity.Note that the oxidation of Sn occurred in the vacuum chamber and under ambient conditions, which caused the degradation of the CsSnI 3 -based powders responsible for the decrease in the PL peak intensity.The decreasing trend in the PL peak intensity with respect to time is similar to the decreasing trend for MAPbI 3 films reported by Mahon et al. [27].However, there exist differences in the observed behavior.Mahon et al. [27] used a continuous-wave laser as the excitation source and focused their study on a short time period of 250 s.It is also noted that continuous illumination of a laser can cause the variation of the concentration of charger carriers and local temperature [28], leading to the change of the PL peak intensity.change, as shown in Figure 1.Such a trend implies that the prepared CsSnI3-based powders are not at a "stable" state.There exist chemical reactions occurring in the CsSnI3based powders over the period of 7 days, as supported by the changes in the molar fractions shown in Tables 1 and 2. The chemical reactions lead to the evolution of individual compounds and result in the decrease in the PL peak intensity.Note that the oxidation of occurred in the vacuum chamber and under ambient conditions, which caused the degradation of the CsSnI3-based powders responsible for the decrease in the PL peak intensity.The decreasing trend in the PL peak intensity with respect to time is similar to the decreasing trend for MAPbI3 films reported by Mahon et al. [27].However, there exist differences in the observed behavior.Mahon et al. [27] used a continuous-wave laser as the excitation source and focused their study on a short time period of 250 s.It is also noted that continuous illumination of a laser can cause the variation of the concentration of charger carriers and local temperature [28], leading to the change of the PL peak intensity.Figure 7a depicts the PL spectra of the CsSnI3-contained powders at different temperatures, from which we determine the peak wavelength and peak intensity at different temperatures.The PL peak wavelength experiences a blue shift from 608 nm to 587 nm when the temperature was increased from 28 °C to 88 °C, and increasing the temperature causes the decrease of the peak intensity.Figure 7b,c shows the temperature dependence of the peak intensity and peak wavelength, respectively.
In general, the variation of the peak intensity of the emission light with temperature can be expressed as [29] a 0 ( / ) ( ) Ae 1 where I0 is the peak intensity at 0 K, Ea is the activation energy, kBT is the thermal energy, and A is a constant.Using Equation ( 3) to fit the data in Figure 7b yields the activation energy of 168.82 meV.For comparison, we include the fitting curve in Figure 7b.Such a Figure 7a depicts the PL spectra of the CsSnI 3 -contained powders at different temperatures, from which we determine the peak wavelength and peak intensity at different temperatures.The PL peak wavelength experiences a blue shift from 608 nm to 587 nm when the temperature was increased from 28 • C to 88 • C, and increasing the temperature causes the decrease of the peak intensity.Figure 7b,c shows the temperature dependence of the peak intensity and peak wavelength, respectively.increasing temperature, supporting Equation (3) with α > 0. The positive value of α implies that there exists electron-phonon interaction, which widens the bandgap of the CsSnI3-contained powders [31].
It must be pointed out that the CsSnI3-dominated powders are unstable after being exposed to the air for a short period.Thus, no experiments were conducted on the thermal stability of the CsSnI3-dominated powders.The photo-responses of the CsSnI3-dominated powders and CsSnI3-contained powders were investigated using a Keithley 2400 sourcemeter.A total of 0.4 mmol of the prepared powders was placed onto a glass substrate.The powders were then shaped to a 1×1 cm 2 shape and connected to a pair of copper electrodes.The photo-response of the powders was evaluated under a voltage sweeping at an increment of 84 mV. Figure 8a presents the I-V curve of the CsSnI3-dominated powder with and without the illumination of white light.Increasing the voltage leads to a nonlinear increase of the current.The illumination of white light significantly increases the current through the powders, revealing the presence of optoelectrical response under the illumination of white light.Figure 8b presents the I-V curve of the CsSnI3-dominated powders under the periodic on-and-off of white light at a time interval of 20 s.It is evident that the illumination of white light reduced the resistance to the current consistent with the results in Figure 8a.The CsSnI3-dominated powders exhibit lucrative photoconductive characteristics, suggesting the potential application in perovskite-based solar cells.In general, the variation of the peak intensity of the emission light with temperature can be expressed as [29] where I 0 is the peak intensity at 0 K, E a is the activation energy, k B T is the thermal energy, and A is a constant.Using Equation (3) to fit the data in Figure 7b yields the activation energy of 168.82 meV.For comparison, we include the fitting curve in Figure 7b.Such a large value of the activation energy suggests that the decrease in the PL peak intensity is ascribed to thermal quenching.According to Yang [30] and Tang et al. [31], the relation between temperature, T, and the bandgap of a semiconductor, E, for |∆T| << T 0 can be formulated to the first-order approximation as with T 0 as a reference temperature, and α as a constant representing the temperature dependence of the bandgap.According to Figure 7b, the bandgap increases linearly with increasing temperature, supporting Equation (3) with α > 0. The positive value of α implies that there exists electron-phonon interaction, which widens the bandgap of the CsSnI 3 -contained powders [31].
It must be pointed out that the CsSnI 3 -dominated powders are unstable after being exposed to the air for a short period.Thus, no experiments were conducted on the thermal stability of the CsSnI 3 -dominated powders.
The photo-responses of the CsSnI 3 -dominated powders and CsSnI 3 -contained powders were investigated using a Keithley 2400 sourcemeter.A total of 0.4 mmol of the prepared powders was placed onto a glass substrate.The powders were then shaped to a 1 × 1 cm 2 shape and connected to a pair of copper electrodes.The photo-response of the powders was evaluated under a voltage sweeping at an increment of 84 mV. Figure 8a presents the I-V curve of the CsSnI 3 -dominated powder with and without the illumination of white light.Increasing the voltage leads to a nonlinear increase of the current.The illumination of white light significantly increases the current through the powders, revealing the presence of optoelectrical response under the illumination of white light.Figure 8b presents the I-V curve of the CsSnI 3 -dominated powders under the periodic on-and-off of white light at a time interval of 20 s.It is evident that the illumination of white light reduced the resistance to the current consistent with the results in Figure 8a.The CsSnI 3 -dominated powders exhibit lucrative photoconductive characteristics, suggesting the potential application in perovskite-based solar cells.8a,b, we can conclude that the CsSnI3-dominated powders have significantly higher photoconductivity than the CsSnI3-contained powders, indicating a higher photon-generated carrier concentration in the CsSnI3-dominated powders than the CsSnI3-contained powders.Note that similar photoconductive behavior was also observed in MAPbI3 by Khenkin et al. [32], who suggested that the variation of dark conductivity is attributed to the phase change and photoconductivity has a non-monotonic dependence on temperature.

Conclusions
Producing lead-free halide perovskites is of practical importance for their applications in solar cells and bioimaging to avoid the detrimental effects of lead to the planet and human health.We have demonstrated that the mechanical grounding of CsI and SnI2 under two different conditions can produce CsSnI3-based powders, which can exhibit dual light emission under two different lights of 365 nm and 785 nm in wavelength.Such a process avoids the use of toxic, organic solvents and can be scaled up to produce Snbased halide perovskites for industrial applications.The prepared CsSnI3-based powders were divided into two groups-one is the CsSnI3-dominated powders, and the other is the CsSnI3-contained powders.Both powders consist of four different chemical compounds

Figure 1 .
Figure 1.Optical images of the prepared powders under white light and UV light of 365 nm in wavelength: (a) CsSnI 3 -dominated powders, and (b) CsSnI 3 -contained powders.A spectrometer (Ocean Optics, FLAME-S-VIS-NIR-ES, Ocean Optics, Orlando, FL, USA) was used to characterize the photoluminescence (PL) of the synthesized CsSnI 3 -based powders under the excitation of UV light of 365 nm in wavelength and a laser of 785 nm in wavelength, respectively.The absorbance of the synthesized CsSnI 3 -based powders was analyzed on a UV-Visible spectrophotometer (EVOLUTION 201, Thermo Fisher scientific, Waltham, MA, USA).The photoconduction of the prepared CsSnI 3 -based powders was examined on a power meter (Keithley 2400, Keithley, Solon, OH, USA) under white light.

1 )▲Figure 3 .
Figure 3. XRD patterns of (a) freshly prepared CsSnI3-dominated powders and the one stored i vacuum chamber for one week, and (b) freshly prepared CsSnI3-contained powders and the o stored under ambient conditions for one week.

Figure 3 .
Figure 3. XRD patterns of (a) freshly prepared CsSnI 3 -dominated powders and the one stored in a vacuum chamber for one week, and (b) freshly prepared CsSnI 3 -contained powders and the one stored under ambient conditions for one week.

Figure 4 .
Figure 4. PL spectra of (a) freshly prepared CsSnI3-dominated and CsSnI3-contained powders under UV light of 365 nm in wavelength, and (b) freshly prepared CsSnI3-dominated and CsSnI3-contained powders under a laser of 785 nm in wavelength.

Figure 4 .
Figure 4. PL spectra of (a) freshly prepared CsSnI 3 -dominated and CsSnI 3 -contained powders under UV light of 365 nm in wavelength, and (b) freshly prepared CsSnI 3 -dominated and CsSnI 3 -contained powders under a laser of 785 nm in wavelength.

Figure 4 .
Figure 4. PL spectra of (a) freshly prepared CsSnI3-dominated and CsSnI3-contained powders under UV light of 365 nm in wavelength, and (b) freshly prepared CsSnI3-dominated and CsSnI3-contained powders under a laser of 785 nm in wavelength.

Figure 6 .
Figure 6.Long-term stability of the CsSnI3-based powders over a period of 7 days: (a) CsSnI3-dominated powders in a vacuum chamber, and (b) CsSnI3-contained powder under ambient conditions.

Figure 6 .
Figure 6.Long-term stability of the CsSnI 3 -based powders over a period of 7 days: (a) CsSnI 3 -dominated powders in a vacuum chamber, and (b) CsSnI 3 -contained powder under ambient conditions.

Figure 7 .
Figure 7. Temperature effects on the PL characteristics of the CsSnI3-contained powders: (a) PL spectra at different temperatures, (b) PL intensity vs. temperature, and (c) photon energy vs. temperature.

Figure 7 .
Figure 7. Temperature effects on the PL characteristics of the CsSnI 3 -contained powders: (a) PL spectra at different temperatures, (b) PL intensity vs. temperature, and (c) photon energy vs. temperature.

Materials 2024 , 13 Figure
Figure8c,d presents the I-V curve of the CsSnI3-contained powder with and without the illumination of white light and the I-V curve of the CsSnI3-contained powders under the periodic on-and-off of white light at a time interval of 20 s, respectively.Similarly, the CsSnI3-contained powders also exhibit photoconductive characteristics.Comparing Figure8c,dto Figure8a,b, we can conclude that the CsSnI3-dominated powders have significantly higher photoconductivity than the CsSnI3-contained powders, indicating a higher photon-generated carrier concentration in the CsSnI3-dominated powders than the CsSnI3-contained powders.Note that similar photoconductive behavior was also observed in MAPbI3 by Khenkin et al.[32], who suggested that the variation of dark conductivity is attributed to the phase change and photoconductivity has a non-monotonic dependence on temperature.

Figure 8 .
Figure 8. Photo-response of the CsSnI3-dominated powders during a voltage sweeping from 0 V to 200 V (a) with and without the illumination of white light and (b) under a 20 s light on-and-off cycle for 385 s; photo-response of the CsSnI3-contained powders during a voltage sweeping from 0 V to 200 V, (c) with and without the illumination of white light and (d) under a 20 s light on-and-off cycle for 385 s.

Figure 8 .
Figure 8. Photo-response of the CsSnI 3 -dominated powders during a voltage sweeping from 0 V to 200 V (a) with and without the illumination of white light and (b) under a 20 s light on-and-off cycle for 385 s; photo-response of the CsSnI 3 -contained powders during a voltage sweeping from 0 V to 200 V, (c) with and without the illumination of white light and (d) under a 20 s light on-and-off cycle for 385 s.

Figure
Figure 8c,d presents the I-V curve of the CsSnI 3 -contained powder with and without the illumination of white light and the I-V curve of the CsSnI 3 -contained powders under the periodic on-and-off of white light at a time interval of 20 s, respectively.Similarly, the CsSnI 3 -contained powders also exhibit photoconductive characteristics.Comparing Figure8c,d to Figure8a,b, we can conclude that the CsSnI 3 -dominated powders have significantly higher photoconductivity than the CsSnI 3 -contained powders, indicating a

Table 1 .
Molar ratios of four compounds in the CsSnI3-dominated powders.

Table 1 .
Molar ratios of four compounds in the CsSnI 3 -dominated powders.